The present invention relates to an integral park brake mechanism in a disc brake actuator.
In the application of disc brakes to motor vehicles it was found difficult to device a mechanism whereby a manually operable parking brake could be applied to a hydraulically operated disc brake system. In some cases the difficulty was met by merely providing disc brakes at the front of the vehicle with conventional drum brakes at the back on which the manually operated mechanical parking brake system acted. In more expensive vehicles disc brakes were provided back and front with separate drums incorporated in the rear disc brakes on which the parking brake acted or separate mechanical calipers dedicated only to parking were provided.
More recently, however, many of the design problems have been overcome and arrangements are available in which a disc brake can be applied either hydraulically or manually through a mechanical system. Such mechanical systems necessitate the use of arrangements providing a high mechanical advantage so that the force that has to be applied to the parking brake lever is not unduly great. Such arrangements are extremely sensitive to the presence of clearances in the system and for this reason most existing designs employ a form of clearance adjuster.
In Australian Patent Specification No. 405,184 there is described a clearance adjuster in which mechanical operation of the brake is effected by means of a stem having a plain cylindrical surface which extends rearwardly from a piston arranged to apply force to a friction pad, the stem passing through a single collar free to tilt about an axis transverse to that of the stem. In manual actuation of the brake, force is applied to the collar through a member having a face inclined at an angle to a plane at right angles to the axis of the assembly so that when the brake is applied mechanically by thrust applied to the member that member applies an offset force to the collar which causes it to lock onto the stem and so transmit the thrust through the stem to a piston and the friction pad. In practice this arrangement was not found to be satisfactory and was never applied commercially.
A problem with the construction described is the inability of the collar to loosen automatically when the tilting load is removed and there is a tendency for the mechanism to stick making it necessary to incorporate a strong return spring to unlock the collar upon removal of the load.
A further problem is that the mechanism acts as a one way adjuster prone to overadjustment to the elastic deflection resulting from hydraulic application of the brake. That is to say that the adjuster is load sensitive.